Various forms of angular velocity sensors have been known hitherto, and from the viewpoint that the entire tuning fork is formed of a ceramic piezoelectric element, as prior art. Japanese Laid-open Patent 3-120415 discloses an oscillating gyro integrally forming two rectangular arms, and a base portion for mutually coupling these arms at their lower ends from a piezoelectric material to form the entire shape into a form of a tuning fork, with the base portion polarized in the direction of Y-axis.
This conventional angular velocity sensor is described below by reference to the drawing.
FIG. 32 is a perspective view of a single-shape tuning fork disclosed in Japanese Laid-open Patent 3-120415.
Directions of polarization are orthogonal, with the base portion in the direction of Y-axis and the driving side oscillating arms in the direction of X-axis. Driving electrodes 3, 4 are partial electrodes of about half of the oscillating arms, and the driving force is 2/8 times as seen from the use of the entire four sides.
Besides, by Coriolis force, the oscillating arms 1, 2 are bent and oscillated in reverse phases in the X-direction, so that a torsional moment about the Y-axis occurs on the base portion 5. Detecting electrodes 6, 7 are to detect torsional oscillation of the base portion 5, and are high in resonance frequency and low in output sensitivity.
Reference numeral 1 is a driving side oscillating arm, and 2 is a monitor oscillating arm for oscillating stably, and the direction of polarization, which is not indicated herein, is supposed to be in the X-direction considering from the function.
In FIG. 32, however, the role functions are divided, that is, the oscillating arms 1, 2 are used for driving, and the base portion 5 for detecting, and although it is only estimation because the mounting or holding method of the base portion 5 is not disclosed, it may be predicted that the oscillation forms are complicated by mixing of (1) oscillating components in the base portion 5 due to driving and oscillation (flexural oscillation in mutually reverse phases in Y-direction), (2) oscillating components in the base portion 5 due to flexural oscillations in mutually reverse phases in X-direction at the time of action of Coriolis force, (3) torsional oscillating components about the Y-axis of the base portion 5, and (4) disturbance noise components from the holding portions. Accordingly, the separation circuit of these four oscillating components is complicated. Since the oscillation analysis of the base portion of the tuning fork is not elucidated by the mechanical vibration engineering of today, its control seems to be difficult. Therefore, since vibration separation is difficult, it may cause malfunction as the gyro in practical aspect. In particular, it is influenced by disturbance noise transmitted from the holding portion, and it was hard to apply in automobiles, etc.
The torsional oscillation is higher in resonance frequency and smaller in oscillation amplitude as compared with flexural oscillation of cantilever, and is hence low in sensitivity. Therefore, drop of output sensitivity was a cause of temperature drift (fluctuation of detection value due to ambient temperature changes when the input angular velocity is 0).
Moreover, since the driving electrodes 3, 4 in FIG. 32 are provided to the leading end in the Y-axis direction of the oscillating arms, according to the vibration theory of tuning fork, 20 to 30% of the leading end functions as floating capacity, not contributing to driving force at all, and only acts to pick up electric noise, and therefore the ratio of detected signal to electric system noise (hereinafter called S/N) was worsened.